The coherent non-differential detection of optical communication signals using phase shift keying (PSK) modulation has long been known to offer superior performance relative to conventional direct detection and differential detection techniques. Coherent non-differential PSK techniques allow operation with a lower optical signal-to-noise ratio (OSNR) to be achieved. Coherent non-differential PSK techniques, by preserving and mapping the optical phase of the PSK signal to the electrical domain, also enable efficient chromatic dispersion (CD) compensation and polarization mode dispersion (PMD) compensation in high-speed optical communication systems using low-speed adaptive digital or analog filtering. To date, however, coherent non-differential PSK techniques have not been deployed in high-speed optical communication systems. Building a high-speed coherent receiver of a reasonable complexity and at a reasonable cost still remains a challenge. Such a high-speed coherent receiver must be able to precisely estimate the relative optical phase between the incoming data signal and the Local Oscillator (LO), as well as to lock the optical phase of the LO to the optical phase of the incoming data signal.
The current state-of-the-art for estimating and/or locking the relative optical phase between the incoming data signal and the LO consists of four different techniques. The first technique involves building the optical phase-locked loop (PLL) function directly in the analog domain, detecting the difference between the LO laser and the incoming data signal, and stabilizing the optical phase and polarization of the LO accordingly. The second technique uses an un-stabilized LO and operates in the digital domain by using high-speed real-time analog-to-digital conversion (ADC) and digital signal processing (DSP) techniques to extract the estimated optical phase in the electrical domain. The third technique also uses an un-stabilized LO, but analog signal processing (ASP) in the radio-frequency (RF) domain to extract the estimated optical phase. The fourth technique involves the transmission of a data signal along with an LO pilot tone which is amplified at the receiver.
A major difficulty associated with coherent non-differential detection techniques is the optical PLL that locks the optical phase of the LO field to the optical phase of the carrier field of the optical signal. It is desirable to have a noiseless optical PLL with zero acquisition time. However, in reality, a balance must always be struck between the noise bandwidth and the acquisition speed. This contributes to optical signal degradation and limits the phase acquisition and tracking speed. In addition, the current state-of-the-art DSP-based approach, considered by many to be the most promising, requires a parallel architecture in the high-speed analog-to-digital converters (ADCs), causing a feedback delay. This necessitates the use of narrow-linewidth source lasers in transmitters and LO lasers with sufficiently long coherence time. Thus, the use of distributed-feedback lasers with a linewidth of ˜1 MHz becomes problematic. Further, the DSP-based approach is limited by the speed of the ADC and DSP devices currently available. For digital systems, the sampling rate is typically 2× the symbol rate. For symbol rates approaching 40 G symbol/s or higher, the required ADC and DSP devices are not yet available. The same narrow-linewidth laser problem exists for the other coherent non-differential detection techniques using either analog optical PLLs or optical phase recovery in the RF domain.
The transmission of a data signal along with an LO pilot tone solves the problem of maintaining a constant optical phase and frequency relationship between the data signal and the LO, as both originate from the same optical source and traverse the same optical path. However, in order to achieve practical coherent detection, the data signal and the LO must be separated at the receiver and the LO pilot tone must be amplified. This is accomplished by one of two mechanisms. The first mechanism is to add the LO pilot tone at the transmitter on a polarization orthogonal to the data signal. The data signal and the LO are then separated at the receiver via polarization tracking and the LO is preferentially amplified. This mechanism, however, precludes the use of polarization multiplexing for data signal capacity doubling. In addition, PMD and polarization-dependent gain/loss reduce polarization orthogonality. The second mechanism is to add an LO pilot tone that is sufficiently separated from the data signal spectrum in frequency (i.e. wavelength), such that the LO pilot tone may be optically filtered out at the receiver and preferentially amplified. The requirement for such frequency separation is set by the ability of the optical filters to provide selective optical filtering and reduces the spectral efficiency of a wavelength-division multiplexed (WDM) optical communication system. In addition, such frequency separation requires the receiver electronics used to operate at a much higher bandwidth, such that the mixing between the data signal and LO spectra is within the electrical bandwidth of the receiver. Further, due to such frequency separation, the optical phase of the LO pilot tone relative to the phase of the data signal depends on CD. As a consequence, the optical phase difference between the LO pilot tone and the data signal may vary over time due to optical fiber temperature-induced dispersion variations. Although such relative optical phase variations are much slower to develop than the relative optical phase variations associated with the external LO schemes, they must still be tracked by an additional PLL.